This application relates to optical data storage, and more particularly, to optical head modules for reading data from or writing data to optical storage media.
Various optical storage devices and systems have been developed. One type of optical storage systems use an optical head to focus a monochromatic optical beam to a small spot on a recording layer of a storage medium for reading or writing data. The optical head may be positioned over the medium by a spacing greater than one wavelength, i.e., in a “far-field” optical configuration, where the optical energy is coupled between the optical head and the medium by light propagation. An optical head with a large numerical aperture can produce a small spot size. The diffraction effect in light propagation, however, limits the numerical aperture to less than unity. The areal density of such an optical storage device, hence, is limited by this diffraction-limited spot size which has a lower limit on the order of one half wavelength.
An optical storage system may also be configured to operate in a “near-field” configuration to achieve an areal density for data storage higher than that of the far-field configuration. In a typical near-field configuration, the optical head is spaced from the optical medium by a distance on the order of or less than one wavelength of the optical energy. The optical coupling between the optical head and the medium, therefore, can be effectuated at least in part by evanescent coupling, with or without light propagation. Some near-field devices couple optical energy through both evanescent coupling and coupling through light propagation. An effective numerical aperture of the optical head in such a near-field configuration can be greater than unity. Hence, a near-field optical storage system can achieve a focused beam spot size much less than one half wavelength and to realize a high areal storage density.
The optical head is an important part of an optical storage system and its properties can significantly affect the overall performance of the system. For example, focusing of an optical beam for reading or writing is primarily determined by the optical head. The optical head also controls other operations of the system, including, among others, recording of data, signal detection, beam tracking on the data tracks, and certain code detection. The design of the optical head and its interfacing with the optical storage medium need special features to accommodate for the near-field nature of interaction and associated technical issues.